Mercury is commonly present in natural gas streams that are produced from an underground formation through a well to surface.
The well fluid is typically split into several liquid and gaseous streams in a natural gas production facility. An initial separation into water, liquid natural gas condensate and gaseous components is typically done in an upstream slug catcher and/or one or more upstream separators, and the various streams are subsequently purified, further separated and/or treated to reach certain specifications.
Removal of mercury from gaseous and also liquid streams from natural gas processing is needed for various reasons. If Hg is not removed at the source, it would end up in products from natural gas such as in Liquified Petroleum Gas (LPG), and after combustion thereof in the environment. Also for this reason, Hg specifications are applicable for various streams dealt with in natural gas processing. There is a price penalty for high Hg concentrations in traded products. Furthermore, Hg removal is often a technical necessity in order to maintain the integrity of downstream equipment. Mercury components can damage metal components, particularly vulnerable for corrosion by Hg are aluminium and aluminium alloy components of processing equipment, such as of a cracker processing natural gas condensate.
In the gas treating section of natural gas processing plant typically an adsorption process is used for mercury removal, in particular adsorption on an activated carbon bed. Such a process cannot be used efficiently for a liquid feedstock such as condensate, especially when asphaltenic material is present as they will choke the adsorption bed.
In liquid condensate, also referred to as Natural Gas Liquid (NGL), mercury can be present in various constitutions, e.g. metallic, ionic, alkyl mercury. It is assumed that these are normally dissolved in atomic, ionic or molecular form, although it cannot be excluded that supramolecular clusters, micro-droplets or the like are formed. These constituents are for example found in sweet condensate, i.e. natural gas condensate that is substantially free of sulphur compounds. In an acidic environment, such as a sour condensate containing substantial amounts of sulphur compounds, such as 500 ppmw S (parts per million by weight), mercury is also or predominantly present as mercury sulphide, which forms solid particulates. Sulphur compounds can in particular be or include H2S.
Known methods for removal of mercury from natural gas condensate often comprise a step of forming mercury sulphide particles, followed by absorption or filtration thereof. Such methods are for example discussed in U.S. Pat. Nos. 4,966,684 and 6,350,372.
U.S. Pat. No. 6,268,543 discusses other adsorbent and extraction methods for mercury removal, and discloses a removal method wherein condensate is contacted with water and sludge, both of which have been contacted in advance with crude oil.
There is a need for an improved process to remove mercury contaminants from natural gas condensate, and for an improved natural gas processing plant.